Been looking at this and the maths works out but it doesn’t ‘feel’ right for some reason and I can’t work out why…
Looking at using the USBizi100 to control a specific transistor.
The transistors are: http://uk.farnell.com/nxp/bc817-40/transistor-npn-45v-0-5a-sot23/dp/1081225
I am trying to calculate the resistor to limit the base current going from the pin to the BC817-40. The USBizi pin is at 3.3V with a max current of 4mA per pin.
Current check: The min hfe of the transistor is 250 with a fully on Ic of 500mA so this should be 2mA from each pin to fully turn it on. I need 32 transistors connected so max package current needs to be kept to a minimum so this would be 64mA so should be fine.
Voltage check: The USBizi runs at 3.3V. The transistor Vbe is 1.2V so this should be fine as the datasheet for the LPC2387 says a minimum of 2.5V when high.
So according to this Rb = (3.3-1.2)/0.002 = 1050Ohm so pick next nearest below = 1k
Would this work? As I say it just doesn’t feel right…
Thanks in advance and my apologies for being a bit dumb on this one.